The almost exclusive sources of energy for combustion engines are fuels produced from petroleum by highly developed chemical-physical processes. All these fuels are mixtures of carbon and hydrogen compounds of differing chemical structure and are well tuned to the combustion process of the engines. Related to the same weight, they have - as compared to other combustible fluids - a high energy density. During combustion, the stored energy is rapidly and directly released and converted into mechanical energy in the engine.
Added to these advantages of liquid fuels there are further special benefits:
Simple and reliable metering
Simple mixing of the atomized fuel with air
Practically no combustion residues with adequate air volume
Constant operational readiness, which - if necessary - can be adapted to the outside temperature
Simple storage and transportation
Network of filling stations throughout the country
These properties, together with the systems of the engine and fuel additives (e.g. ignition accelerators), make it possible to adapt fuels to combustion conditions and to economically exploit their energy. Currently fuels of biogenic origin – so-called alternative fuels – are increasingly being added to fossil fuels.
On a worldwide basis, fuels have differing qualities. The quality of the fuel is critical for the performance and the exhaust emissions of the vehicles. Consequently, the fuel grade should be harmonized to a suitable level in accordance with the vehicle requirements. In order to achieve these goals, the "World Wide Fuel Charter" (WWFC) was created by the global automotive industry. This defines suitable objections of fuel characteristics for the various emissions and CO2 requirements. The WWFC is revised at regular intervals and adapted to reflect the state of the art.